In studies with macrophage cell lines, we have been able to induce Fc-mediated phagocytosis in a line lacking it by short exposure to DMSO. We have also selected and characterized mutants that are reduced in generalized phagocytosis. We have studied Salmonellae that differ in pathogenicity for mice solely because of a difference in lipopolysaccharide (LPS) side chains, and found that the different polysaccharides differ in degree of activation of complement via the alternative pathway in an antibody-independent fashion. The degree of activation determines the subsequent ingestion of the bacteria by macrophages, presumably via the Fc receptor. It is likely that this mechanism of pathogenicity is operative in vivo. LPS is heterogeneous in side chain length, which varies from 0 to more than 40 repeating units per molecule. We find heterogeneity is maintained in mutants almost completely blocked in ability to make side chain, indicating preference of biosynthetic enzymes for certain rather long lengths. We have also demonstrated and characterized a long polysaccharide with the composition of LPS side chain but not attached to lipid A, loosely attached to the cell surface, that probably accounts for certain false capsule responses.